MFJ-269 Instruction Manual HF/VHF/UHF SWR Analyzer
36
Vf entry of 1.0) is 75 feet, and the transmission line is actually 49.5 feet long, the velocity factor is 49.5 divided
by 75, for a result of 0.66 Vf.
NOTE: The far end of the line can be either
open circuited
or
short circuited.
The line can not be terminated in
any impedance other than an open or short.
To confirm reliability, make two or more groups of measurements on different starting frequencies at least one
octave apart. If measured distances agree, they are almost certainly very reliable.
Use the following method:
1.)
Using procedures in 5.5 Advanced 2, measure distance to fault with Vf set at 1.00.
2.)
Measure the physical length of the line in feet.
3.)
Divide the actual physical feedline length by the display reading.
Example:
27 feet (actual physical length) divided by 33.7 feet (measured electrical length) equals .80. The
velocity factor is .80 or 80%.
4.)
If the analyzer is now set to Vf= .80 and the line measured, the result should be the correct physical length.
7.3 Impedance of Transmission Lines or Beverage antennas
The impedance of transmission lines between a few ohms and 1500 ohms can be directly measured with the MFJ-
269. Lines of higher impedance can be measured if a broadband transformer or resistance is used to extend the
MFJ-269’s range. Select any measurement mode that indicates resistance (R=) and reactance (X=).
1.) If a balanced line is used, operate the MFJ-269
only
from internal batteries. Keep the MFJ-269 a few feet
away from other conductors or earth, and do not attach any wires (other than the feedline) to the unit. Use the
ANTENNA connector’s shield for one lead and its center pin for the other. Two wire balanced lines
must
be
suspended in a fairly straight line a few feet away from metallic objects or ground.
2.) Coaxial lines can lay in a pile or coil on the floor. Internal or external power can be used, and the MFJ-269
can be placed on or near large metallic objects with no ill effects. Coaxial lines connect normally, with the shield
grounded.
3.) Beverage antennas can be directly connected to the MFJ-269.
Using fixed resistances:
1.
Terminate the line or antenna in a non-inductive resistance somewhere around the expected value.
2
. Connect the transmission line or antenna directly to the MFJ-269 "ANTENNA" connector. Adjust the
frequency (near the expected operating frequency) until the lowest resistance and lowest reactance is
measured.
3
. Record the impedance value.
4
. Adjust the frequency until the highest resistance and
lowest
reactance is measured.